Because of the excellent mechanical properties, carbon fiber reinforced plastics (hereinafter abbreviated as CFRP) are being used as structural parts in a variety of fields and also used in sporting and leisure goods. Although various matrix resins are used in these applications, epoxy resins are extensively used owing to their attractive characteristics that they have superior mechanical properties, that no volatile ingredients are released during cure, that the shrinkage during cure is only little, and that the adhesion to carbon fibers is excellent.
However, with the recent trend toward weight and thickness reduction in CFRP products, defective products resulting from warpage after molding are increasing in number in the production of molded products such as pipes and this has been becoming a problem. In particular, in the field of fishing rod, a large number of warped products should be reformed or discarded, so that the production of fishing rods involves considerable waste from the economical and production efficiency standpoints. To eliminate such a problem, various measures have been taken such as, for example, varying the production conditions so as to diminish unevenness in wall thickness in fiber-reinforced plastics, especially those in the form of pipe, or to attain proper dispersion of carbon fibers, and decreasing the crosslink densities of matrix resins and diminishing residual thermal stresses generating at interfaces by incorporation of plasticizers.
The thermal stress .sigma. is expressed by the equation .sigma.=E(.alpha..sub.R -.alpha..sub.CF) (Tg-T.sub.O) (wherein E represents the elastic modulus of matrix resin, .alpha..sub.R and .alpha..sub.CF represent the linear expansion coefficient of the resin and that of the carbon fibers, respectively, Tg is the glass transition temperature of the resin, and T.sub.O is the temperature at which stress is measured). The above equation indicates that the thermal stress .pi. is decreased if E, .alpha..sub.R, or Tg can be lowered. Based on the above, improvements of matrix resins are being made in various ways.
For example, it has been proposed to disperse rubber particles in a matrix to thereby lower E while retaining Tg, as disclosed, for example, in JP-A-58-108220. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Further, JP-A-1-272621to JP-A-1-272624 propose to use specific polyether-modified silicone polymers to lower .alpha..sub.R and E without decreasing Tg. Both methods, however, are defective in that since the elastic modulus of resin (E) is lowered, the resulting CFRP products inevitably have insufficient mechanical properties. On the other hand, U.S. Pat. No. 4,480,082 and European Patent Application 0,103,392A discloses improvement of resin properties by use of a compound similar to that employed in the present invention, but there is no description therein teaching or suggesting use of a resin composition, such as that of the present invention, for diminishing the warpage of CFRP cross laminates or improving the strengths thereof. Thus, a simple and economical method of diminishing the warpage of carbon fiber reinforced plastic products while retaining their mechanical strengths has not yet been accomplished.